A network tap is a hardware device with takes a copy the network traffic/frames and provides them to a monitoring device. A network tap has at least three ports: a first network port for connecting a first network device, a second network port for connecting a second network device and a third network port for connecting a monitoring device. The tap is inserted between the first network device and the second network device and allows all traffic to pass through it unimpeded in real-time, but further copies this traffic to the third network port, enabling the monitoring device to monitor this traffic. A network tap should have the lowest impact on the monitored network devices. The network tap should not cause connection outage or cause performance (e.g. bit error rate) degradation. Network taps are commonly used for networking intrusion detection and network probes, amongst others.
Network taps exist for both electromagnetic signals, e.g. transported by twisted pair cables, and fiber optic signals. U.S. Pat. No. 7,542,681 B2, for example, describes such a fiber optic network tap. This tap includes a first port for connecting to a pluggable optoelectronic module such as an optical transceiver, a link port for connecting to an optical link configured to receive and send data to the optical transceiver, and a tap port for relaying diverted optical data to a storage and/or analyzing device. Couplers are used to split the optical signals entering the tap from the first port and/or the link port such that a usable portion of the optical signal(s) can be stored and/or analyzed.
For decades, fiber light splitting has been the preferred method for fiber line data monitoring. With this passive tapping method, i.e. a method not requiring a powered circuit, a part of the light is taken from the fiber optic communication line for monitoring purposes. The splitter adds an attenuation (in dB) on the line, deducting from the optical budget (which can be expressed as the difference in dB between transmitter power and the receiver sensitivity).
Very often, a light splitting ratio is selected, in order to have the least impact on the tapped communication line, and a minimal amount of light is taken to be able to monitor it. Because the Bit Error Rate characteristic is related to the light level at the receiver. One of the cheapest and most reliable components offering light splitting with a factory selectable split ratio is the FBT (Fused Biconical Tapered) splitter.
With the increase of communication speeds (10G, 40G, 100G, 400G), fiber optic communications are more sensitive to noise, have tighter optical budget. Because of the line distances, the optical budget is sometimes too weak to be able to monitor the line with traditional methods, introducing an extra attenuation to the line would disturb the communication line, making it unstable or degrading the performances.